2,5-(Hydroxymethyl)furaldehyde, also known as 2,5-(hydroxymethyl)-furfural (HMF), has many important industrial and commercial applications, largely due to its many functional groups and ability to serve as a precursor in many polymerization reactions. HMF, for example, is a suitable starting source for the formation of various furan monomers required for the preparation of non-petroleum-derived polymeric materials. HMF, as well as other 2,5-disubstituted furanic derivatives, also has great potential for use in the field of intermediate chemicals produced from regrowing (i.e., renewable) resources. Also due to its various functionalities, HMF may be used to produce a wide range of products, including, but not limited to, polymers, solvents, surfactants, pharmaceuticals, and plant protecting agents. The structure of HMF is shown below:

The use of HMF and other furfural derivatives may be compared with the use of corresponding benzene-based macromolecular compounds. In order to be cost-effective and compete in this market, HMF must be produced at competitive prices. The production of HMF has been studied for years, but an efficient and cost-effective method of producing HMF in high yields has yet to be found. HMF is primarily produced from the dehydration reaction of a carbohydrate compound, particularly monosaccharides, including glucose and fructose. After dehydration, complications can arise, such as the rehydratiing of HMF, often yielding the by-products levulinic acid and formic acid. Another competing side reaction is the polymerization of HMF and/or fructose to form humin polymers.
Hexoses are the preferred carbohydrate source from which HMF is formed. Fructose is the preferred hexose used for the dehydration reaction to form HMF. This is in part because fructose has been shown to be more amenable to the dehydration reaction. The fructose structure is shown below:

Fructose, however, is more expensive than other hexoses, such as glucose (dextrose), and maltose, for example. Early processes and procedures for the production of HMF focused on the use of crystalline fructose, but its widespread use is prevented by its high cost. Other sources of fructose, including high-fructose corn syrup (HFCS), have been used to produce HMF and other furan derivatives. Szmant and Chundery used high fructose corn syrup as a starting material in forming HMF, as disclosed in a 1981 article in J. Chem. Tech. Biotechnol., 31, (pgs. 135-145). Szmant and Chundry used a variety of carbohydrates as starting material, but designed reaction conditions specific to each carbohydrate source. For example, they used a boron trifluoride catalyst (BF3 Et2O) with DMSO as a solvent in the conversion of HFCS to HMF, but utilized different catalyst/solvent combinations with different starting materials. Use of BF3 Et2O as a catalyst is not economically practical since it cannot be recovered and re-used. Furthermore, Szmant and Chundry required the use of a Pluronic emulsifier to suppress foaming. They also required bubbling of nitrogen to suppress oxidation and the use of DMSO as a solvent, which is not easily separable from the HMF product, and therefore creates difficulties with product recovery. It remains desirable, therefore, to develop an industrially practicable process for producing HMF in high purit.
U.S. Pat. No. 6,706,900 to Grushin et al. (Grushin '900) also discloses the dehydration of fructose in the form of high-fructose corn syrup, to form HMF as an intermediate; but this process is performed in the context of forming diformylfuran (DFF), also known as 2,5-dicarboxaldehyde. The reaction proceeds in an aqueous environment, and the HMF that is formed is not isolated from the reaction mixture, but rather is directly converted to DFF without an isolation step. The reaction conditions of Grushin '900 are therefore not constrained by considerations of product yields of HMF, as it is formed as an intermediate that is not isolated as a product. More importantly, from a practical commercial standpoint, Grushin '900 is not constrained by considerations of isolating HMF from the product mixture. An efficient method for producing HMF in desirable yields and sufficiently high purity from a natural and industrially convenient carbohydrate source or mixed carbohydrate source has yet to be found.
Water has been used as a solvent of choice in dehydration reactions forming HMF because of the solubility of fructose in water. Aqueous conditions, however, have proven to deleteriously affect the dehydration reaction of fructose to HMF in a variety of ways. Aqueous conditions have led to decreased yield of HMF as low selectivity for the dehydration reaction has been demonstrated. Furthermore, solvation of protons in water highly reduces the catalytic activity for the dehydration reaction. Low selectivity of the dehydration reaction simultaneously leads to increased polymerization reactions and humin formation, which also interfere with the synthesis of HMF.
In an attempt to solve such problems associated with aqueous systems, one proposed solution involves an improvement by simultaneously extracting HMF after the dehydration reaction. A similar attempt to improve yields involves the adsorption of HMF on activated carbon. The key factor in these processes is a rapid removal of HMF from the acidic medium in which it is formed. However, these systems generally suffer from high dilution or partially irreversible adsorption of HMF. These problems have been addressed by a number of different methods, including but not limited to selecting a proper choice of solvents, as disclosed in our co-pending U.S. Provisional Application Ser. No. 60/635,406.
In another attempt to solve the problems of aqueous systems, an organic solvent may be added to the aqueous solution, such as, for example, butanol or dioxane. Such systems, however, present a difficulty in that rehydration of HMF is common and ether formation of HMF occurs with the solvent if alcohols are employed. High yields of HMF, therefore, were not found with the addition of these organic solvents. In a further attempt to provide an adequate solvent system, aqueous solvent mixtures and anhydrous organic solvents have also been employed to ensure favorable reaction conditions. Examples of anhydrous organic solvents used include dimethylformamide, acetonitrile, dimethylsulfoxide, and polyethylene glycol.
Dimethylsulfoxide (DMSO), for example, has been extensively studied and employed as a solvent in the dehydration reaction to form HMF. Improved yields of HMF have been reached with ion exchangers or boron trifluoride etherate as a catalyst, and even without any catalyst. DMSO presents a problem, however, in that recovery of HMF from the solvent is difficult.
Furthermore, although dehydration reactions performed in solvents with high boiling points, such as dimethylsulfoxide and dimethylformamide, have produced improved yields, the use of such solvents is cost-prohibitive, and additionally poses significant health and environmental risks in their use. Still further, purification of the product via distillation has not proven effective for a variety of reasons. First of all, on long exposure to temperatures at which the desired product can be distilled, HMF and impurities associated with the synthetic mixture tend to be unstable and form tarry degradation products. Because of this heat instability, a falling film vacuum must be used. Even in use with such an apparatus however, resinous solids form on the heating surface causing a stalling in the rotor, and the frequent shutdown resulting therefrom makes the operation inefficient.
Catalysts may also be used to promote the dehydration reaction of fructose to HMF. Some commonly used catalysts include cheap inorganic acids, such as H2SO4, H3PO4, HCl, and organic acids such as oxalic acid, levulinic acid, and p-toluene sulfonic acid. These acid catalysts are utilized in dissolved form, and as a result pose significant difficulties in their regeneration and reuse, and in their disposal. In order to avoid these problems, solid sulfonic acid catalysts have also been used. Solid acid resins, however, are limited in use by the formation of deactivating humin polymers on their surfaces under conditions taught in the art. Other catalysts, such as boron trifluoride etherate, can also be used. Metals, such as Zn, Al, Cr, Ti, Th, Zr, and V can be used as ions, salts, or complexes as catalysts. Such use has not brought improved results, however, as yields of HMF have continued to be low. Ion exchange catalysts have also been used, but have also delivered low HMF yields under conditions taught in the art, and further limit the reaction temperature to under 130° C., which accordingly limits the yield.
HMF derivatives may be more stable and easier to synthesize than HMF. Derivatives of particular interest include a compound principally derived by the reduction of HMF, 2,5-bis-(hydroxymethyl)tetrahydrofuran (THF-diol), and a compound principally derived by the oxidation of HMF, 2,5-furandialdehyde (2,5-FDA). Because an economically feasible way to produce HMF had not been discovered (prior to the discovery disclosed in U.S. Provisional Application No 60/635,406), there also been a corresponding lack of interest in the production of these HMF derivatives. The difficulties associated with synthesizing HMF increase the cost of obtaining HMF, and there has been a corresponding lack of the starting material to synthesize THF-diol and 2,5-FDA. Improved methods of synthesizing HMF can be found in our commonly-owned co-pending U.S. Provisional Patent Application Ser. No. 60/635,406, filed Dec. 10, 2004. The structures of HMF and the corresponding derivatives are shown below:

THF-diol is known to be used as a solvent, softener, humectant, and in the synthesis of plasticizers, resins, surfactants, and agricultural chemicals. THF-diol is also known to be used in pharmaceutical applications. THF-diol is typically prepared by Raney nickel reduction of HMF or dimethyl furan-2,5-dicarboxylate. These procedures, however, have not produced suitable yields, and are performed under extreme reaction conditions, both of which make the synthesis unattractive industrially. The reduction of HMF over Raney nickel, for example, is performed at 75 atmospheres and at 130° C. and has not provided satisfactory product yields. Furthermore, because HMF is difficult to obtain commercially, the synthesis of THF-diol from HMF had not been considered a viable industrial alternative. Still further, viable methods of purifying THF-diol have also not been reported, which further has discouraged the search for an efficient synthetic route for making THF-diol.
U.S. Pat. No. 3,083,236 to Utne et al. for example (Utne '236) discloses the synthesis of a number of derivatives from HMF, including THF-glycol. THF glycol is produced mainly as a by-product of other derivatives when using copper chromite as a catalyst under high pressure (approximately 5,000 psi), and Utne '236 also discloses the use of a Raney nickel catalyst, or palladium on charcoal without copper chromite to produce THF-glycol. The reaction conditions, however, require a substantial time period to promote the synthesis.
The synthesis of 2,5-furandialdehyde from HMF has also been attempted, but its industrial application has been limited due to extended reaction times necessary to promote the reaction, harsh reaction conditions (high temperatures), and poor yields, as well as the lack of an industrially acceptable method of synthesizing HMF.
The oxidation of HMF using vanadium catalysts and complex procedures involving bubbling air through the reaction mixture for 24 hours using dimethyl sulfoxide (DMSO) has been thought to be necessary to synthesize 2,5-furandialdehyde. Furthermore, recovery and purification methods for the 2,5-furandialdehyde have been severely limited so that the synthesis has not heretofore been effectively implemented industrially. In sum, economically feasible reactions have not yet been found to synthesize the HMF derivatives THF-diol or 2,5-FDA. Accordingly, a need remains for such reactions.
HMF derivatives have many known uses. In addition, a novel use for such derivatives disclosed herein is as an ingredient of a coating composition. Typical mixtures of liquid paint coatings, including interior latex paint, are dilute solutions of organic resins with organic or inorganic coloring agents, and additives and extenders dissolved in an organic solvent. The organic solvent gives the coating solution the necessary viscosity, surface tension, and other properties necessary to allow application of a smooth layer of the solution. Typical coating solvents, such as ethylene glycol, have high volatility, and contribute substantially to the coatings' volatile organic contents (VOC). VOC is commonly measured in paints, and high VOC is undesirable, as highly volatile organic solvents contribute to lingering paint smells, and may emit fumes arguably contributing to such maladies as “Sick Building Syndrome,” “Danish Painters Syndrome,” asthma, allergies, and other chemical sensitivities. Accordingly, there is a need for environmentally friendly paints and other coating compositions having reduced volatile organic content, as well as industrially convenient methods of making ingredients that may be included in coating compositions to reduce VOC of such compositions.